1. Field of the Invention
The present invention concerns in general the cooling of a gradient coil as used in magnetic resonance tomography (MRT). The present invention in particular concerns the application of cooling tubes as well as a sealing compound for better heat dissipation at the gradient coils.
2. Description of the Prior Art
MRT is based on the physical phenomenon of nuclear magnetic resonance and has been successfully used as an imaging method for over 15 years in medicine and biophysics. In this examination modality, the subject is exposed to a strong, constant magnetic field. The nuclear spins of the atoms in the subject, which were previously randomly oriented, thereby align. Radio-frequency energy can now excite these “ordered” nuclear spins to a specific oscillation (resonance frequency). In MRT, this oscillation generates the actual measurement signal (RF response signal) which is acquired by means of appropriate reception coils.
For the image reconstruction, the exact information about the origination location of the RF response signal (spatial information or spatial coding) is a requirement. This spatial information is acquired via magnetic auxiliary fields (magnetic gradient fields) relative to the static magnetic field along the three spatial directions. In comparison to the main field, these gradient fields are small and are generated in the patient opening of the magnet by additional conductor coils. The entire magnetic field, and therewith the resonance frequency as well, is different in each volume element due to these gradient fields. If a definite resonance frequency is emitted, only the atomic nuclei that are at a location at which the magnetic field satisfies a corresponding resonance condition can be excited. Suitable modification of the gradient fields enables it to definitely shift the location of such a volume element in which the resonance condition is satisfied, and thus to scan the desired region.
MRT allows a free selection of the layer to be imaged, so slice images of the human body can be acquired in all directions. MRT today makes use of high gradient capacities that enable an excellent image quality with measurement times in the range of seconds and minutes.
The constant technical development of the components of MRT apparatuses and the introduction of faster imaging sequences continuously makes more fields of use in medicine available to MRT. Real time imaging for the support of minimally-invasive surgery, functional imaging in neurology and perfusion measurement in cardiology are only a few examples.
The basic design of one of the basic components of such an MRT apparatus is shown in FIG. 7. It has a basic field magnet 1 (for example an axially superconducting air-core coil magnet with active scatter field shielding) that generates a homogenous magnetic basic field in one direction, for example the z-direction, in an inner chamber. The superconducting magnet 1 internally has superconducting coils that are located in liquid helium. The basic field magnet 1 is surrounded by a two-part shell that is normally made from stainless steel. The inner shell, which contains the liquid helium and also serves in part as a winding body for the magnet coils, is suspended over weakly heat-conducting glass fiber-reinforced rods (rods) on the outer shell, which is at room temperature. Vacuum prevails between inner and outer shell.
The cylindrical gradient coil 2 inside the basic field magnet 1 is concentrically inserted inside the support tube by means of support elements 7. The support tube is externally bounded by an outer shell 8, internally by an inner shell 9.
The gradient coil 2 has three windings that generate respective gradient fields perpendicular to one another and proportional to the respectively applied current. As shown in FIG. 8, the gradient coil 2 has an x-coil 3, a y-coil 4 and a z-coil 5 that are respectively wound around the coil core 6 and thus generate gradient fields in the directions of the Cartesian coordinates x, y and z.
The x-coil 3 and the y-coil 4 are of a type known as saddle coils that overlap in the edge regions, are generally rotated counter to one another by 90° relative to the z-axis. The z-coil 5 is a conventional Maxwell coil.
Since the magnetic resonance frequency is directly dependent on the magnetic field strength, the same field strength should prevail in the defined measurement volume at each point in this volume. This is important for the spatial resolution or imaging and for the reproducibility of frequency spectra in spectroscopic imaging, in which field distortions caused by the susceptibility of the measurement subject must be subsequently corrected.
Among others, two shimming techniques are known for the homogenization of the basic field magnet:
1. A further orthogonal coil system with current flowing therein is located within the gradient coil 2, with which it is possible to homogenize the base field magnet. These additional correction coils 10 (shim coils) (FIG. 9) serve to compensate field inhomogeneities of a higher order and are therefore of very complicated design.
2. For homogenization of the basic field magnet, a suitable arrangement of iron plates 1 (shim plates) (FIG. 9) to be mounted in the magnet bore—meaning inside the gradient coil of between gradient coil and basic field magnet—is calculated with a field calculation program. An advance measurement of the field distribution serves as a requirement for the calculation. After the mounting, another calibration measurement is implemented. This event must be repeated multiple times before a satisfactory shim result is achieved.
While the technique under point 1 represents an active shim, the technique under point 2 is designated as a passive shim.
Both gradient coils and shim coils are produced from lacquer-insulated copper flat wire 13 that exhibits a rectangular cross-section according to FIG. 6. The wire 13 is wound to the coil in special winding shapes (two-dimensional with saddle coils) and adhered to a carrier 12 (FIG. 6). This is subsequently curved into the shape corresponding, for example, to the saddle coil and mounted on the gradient coil 2. In the next step, the coils are connected by the coil ends being soldered with current supply cables and thus are connected with the gradient power supply or the shim power supply.
It is the object of the gradient power supply to generate (with precise amplitude and timing) current pulses corresponding to the sequence programmed in the pulse sequence controller. The necessary currents are at approximately 250 A, the current rise rates are in the range of 250 kA/s.
Under such conditions, a great deal of heat (that can only be dissipated by active cooling) is created due to electrical power loss in the gradient coils and in the shim coils. According to the prior art, this is realized (as shown in FIG. 6) by plastic or copper heat exchangers 18 that are initially brought into the best possible contact with the respective coils. Coils 13 and heat exchangers 18 are subsequently provided with a chilled casting 16 to achieve the necessary mechanical stability.
This chilled casting 16 is normally formed of an epoxy resin hardened with acid anhydride, with quartz powder as a filler, and exhibits the necessary high values for insulation resistance and partial discharge stability. The heat conductivity is, however, only at 0.8 W/mK, such that the heat dissipation from the coil 13 ensues only insufficiently at the perimeter of the heat exchanger 18. Additionally, the contact area between coil 13 and heat exchanger 18 is very small, since due to its geometry—cooling coil with annular cross-section—only a line-shaped bearing surface results. A more effective heat transfer is, however, primarily thereby prevented since the material of the present plastic heat exchanger 18 is a commercially available thermoplastic synthetic with a heat conductivity of only approximately 0.2 W/mK. Due to the wall thickness necessary based on the pressure resistance, this represents the decisive heat resistance.
A copper heat exchanger—as it is presently used—in fact overall forms a larger contact area with the coil to be cooled, since the cross-section of the copper tube is fashioned rectangular; however, it is exhibits other non-negligible disadvantages: the design of such a copper heat exchanger is very elaborate in terms of construction, since the curvature of a rectangular (in cross-section) tube involves cross-section modifications. It must therefore be soldered at reversal points, which leads to quality risks and increased production costs. A further large disadvantage of an electrically-conductive heat exchanger in the form of a copper heat exchanger is the fact that magnetic field changes generated by the gradient coils induce in it eddy currents that are transduced into heat (due to the ohmic resistance) and heat the cooling tubes.
Furthermore, it is known to press or to mount the gradient coils (saddle coils and Maxwell coil) onto a highly heat-conductive ceramic cylinder.
WO/94/20862 discloses a gradient coil system with a double-walled ceramic cylinder made from silicon nitride, aluminum oxide or, respectively, a mixture thereof with markedly high heat conductivity (>3 W/mK). Coolant fluid (for example water) flows through the plenum, such that a flowing water jacket is created via which the heat of the cylinder and the gradient coils in contact with the cylinder is dissipated.
Due to increasing requirements for the capacity of modern gradient systems, this also means an increase in the heat to be dissipated. If the heat is not sufficiently dissipated, the sealing compound, for example, heats above the glass transition temperature at which it changes from mechanically solid into the rubber-elastic state, which leads to drastic changes of the mechanical properties. The heating of the sealing compound would likewise involve a heating of the shim iron plates and, as a result, a distortion of the magnetic fields, which would negatively affect the image quality of the MR exposures. In particular in magnetic resonance tomography systems for whole-body examinations, a good cooling of the gradient and shim coils is particularly necessary because the patient lies inside the gradient coil subassembly (fashioned tube-shaped) and a significant heating would represent an unacceptable stress of the patient.